libfdt: Add support for appending the values to a existing property

[Jon Loeliger]

> From: Minghuan Lian <Minghuan.Lian at freescale.com>
> 
> Some properties may contain multiple values, these values may need
> to be added to the property respectively. this patch provides this
> functionality. The main purpose of fdt_append_prop() is to append
> the values to a existing property, or create a new property if it
> dose not exist.
> 
> Signed-off-by: Minghuan Lian <Minghuan.Lian at freescale.com>
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
> --- 
> 
> Jon,
> 
> Minghuan Lian sent this to the list ages ago, but it got stuck for
> months in a mailing list mishap.  It more recently resurfaced, I made
> some small cleanups and added testcases.  Please apply.

Ooo, sorry. So, yeah...

Applied.

Thanks,
jdl